I would get the highest possible spec-machine of the G4 generation I am interested in.
Weather 1.5GB or 2GB RAM doesn't seem to be that much of an issue, though TenFourFox (same as newer Firefox versions on Intel Macs) LOVES much RAM and if you plan to use the Mac for secure internet browsing you won't get arround TenForFox.
Webkit browsers like safari are more volatile, there is however an overhauled version of LeopardWebkit-Browser that can be used. Oh, yes, if you somewhen want to try Mac OS 10.5 Leopard, then you need at least 867MHz (unlss you want to hack it install it. A Dual 500MHz or Dual 533MHz though runs ok with 10.5, I heard, if you get 10.5 installed via hacks).
Did you compare the prices to sold listings on Ebay to get a feel for what "decent" prices are?
Also mind that some models have a Hard Drive limit of 128GB per drive. This goes away somewhere in the 2001 Quicksilver line, to know exactly, look at this page, it lists Processor speeds and the motherboard part number.
http://www.macos9lives.com/mac os 9 lives_003.htm There is at least one Quicksilver model that has the same CPU speed, but different motherboard.
(Of course you can get arround the HDD limit buying PCI and PCI-X cards, but that is another topic).
Also, if you get the question mark at start up and a HDD is in it, try reseating the IDE cable (I know this sounds stupid...
).